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Versions: 00 01 02 03 04 05                                             
IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Best Current Practice                           G. Gont
Expires: September 1, 2017                                  SI6 Networks
                                                         M. Garcia Corbo
                                                                 SITRANS
                                                       February 28, 2017


                   IPv6 Address Usage Recommendations
            draft-gont-6man-address-usage-recommendations-01

Abstract

   This document analyzes the security and privacy implications of IPv6
   addresses based on a number of properties such as address scope,
   stability, and usage type.  It analyzes what properties are desirable
   for some popular scenarios, and provides advice regarding the
   configuration and usage of such addresses.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 1, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Predictability Considerations . . . . . . . . . . . . . . . .   3
   4.  Address Scope Considerations  . . . . . . . . . . . . . . . .   3
   5.  Address Stability Considerations  . . . . . . . . . . . . . .   4
   6.  Usage Type Considerations . . . . . . . . . . . . . . . . . .   5
   7.  Advice on IPv6 Address Configuration  . . . . . . . . . . . .   6
   8.  Advice on IPv6 Address Usage  . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     12.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   IPv6 hosts typically configure a number of IPv6 addresses, which may
   differ in multiple aspects, such as address scope and stability (e.g.
   stable addresses vs. temporary addresses).  For example, a host may
   configure one stable and one temporary address per each
   autoconfiguration prefix advertised on the local network.  The
   addresses to be configured typically depend on local system
   configuration, with the aforementioned configuration being static and
   irrespective of the network the host attaches to.

   There are three parameters that affect the security and privacy
   properties of an address:

   o  Scope

   o  Stability

   o  Usage type (client-like "outgoing connections" vs. server-like
      "incoming connections")

   Section 3, Section 4, Section 5, and Section 6 discuss the security
   and privacy implications (and associated tradeoffs) of the scope,
   stability and usage type properties of IPv6 addresses, respectively.





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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Predictability Considerations

   Predictable IPv6 addresses result in a number of security and privacy
   implications.  For example, [Barnes2012] discusses how patterns in
   network prefixes can be leveraged for IPv6 address scanning.  On the
   other hand, [RFC7707], [RFC7721] and [RFC7217] discuss the security
   and privacy implications of predictable IPv6 Interface Identifiers
   (IIDs).

   Given the aforementioned previous work in this area, and the formal
   specification update produced by [RFC8064], we expect (and assume in
   the rest of this document) that implementations have replaced any
   schemes that produce predictable addresses with alternative schemes
   that avoid such patterns (e.g., RFC7217 in replacement of the
   traditional SLAAC addresses that embed link-layer addresses).

4.  Address Scope Considerations

   The IPv6 address scope can, in some scenarios, limit the attack
   exposure of a node as a result of the implicit isolation provided by
   a non-global address scope.  For example, a node that only employs
   link-local addresses may, in principle, only be exposed to attack
   from other nodes in the local link.  Hosts employing only Unique
   Local Addresses (ULAs) may be more isolated from attack than those
   employing Global Unicast Addresses (GUAs), assuming that proper
   packet filtering is enforced at the network edge.

   The potential protection provided by a non-global addresses should
   not be regarded as a complete security strategy, but rather as a form
   of "prophylactic" security (see
   [I-D.gont-opsawg-firewalls-analysis]).

   We note that the use of non-global addresses is usually limited to a
   reduced type of applications/protocols that e.g. are only meant to
   operate on a reduced scope, and hence their applicability may be
   limited.

   A discussion of ULA usage considerations can be found in
   [I-D.ietf-v6ops-ula-usage-considerations].






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5.  Address Stability Considerations

   The stability of an address has two associated security/privacy
   implications:

   o  Ability of an attacker to correlate network activity

   o  Exposure to attack

   For obvious reasons, an address that is employed for multiple
   communication instances allows the aforementioned network activities
   to be correlated.  The longer an address is employed (i.e., the more
   stable it is), the longer such correlation will be possible.  In the
   worst-case scenario, a stable address that is employed for multiple
   communication instances over time will allow all such activities to
   be correlated.  On the other hand, if a host were to generate (and
   eventually "throw away") one new address for each communication
   instance (e.g., TCP connection), network activity correlation would
   be mitigated.

      NOTE: the use of constant IIDs (as in traditional SLAAC) result in
      addresses that, while not constant as a whole (since the prefix
      changes), contain a globally-unique value that leaks out the node
      "identity".  Such addresses result in the worst possible security
      and privacy implications, and their use has been deprecated by
      [RFC8064].

   Typically, when it comes to attack exposure, the longer an address is
   employed the longer an attacker is exposed to attacks (e.g. an
   attacker has more time to find the address in the first place
   [RFC7707]).  While such exposure is traditionally associated with the
   stability of the address, the usage type of the address (see
   Section 6) may also have an impact on attack exposure.

   A popular approach to mitigate network activity correlation is the
   use of "temporary addresses" [RFC4941].  Temporary addresses are
   typically configured and employed along with stable addresses, with
   the temporary addresses employed for outgoing communications, and the
   stable addresses employed for incoming communications.

      NOTE: Ongoing work [I-D.gont-6man-non-stable-iids] aims at
      updating [RFC4941] such that temporary addresses can be employed
      without the need to configure stable addresses.

   We note that the extent to which temporary addresses provide improved
   mitigation of network activity correlation and/or reduced attack
   exposure may be questionable and/or limited in some scenarios.  For
   example, a temporary address that is reachable for, say, a few hours



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   has a questionable "reduced exposure" (particularly when automated
   attack tools do not typically require such a long period of time to
   complete their task).  Similarly, if network activity can be
   correlated for the life of such address (e.g., on the order of
   several hours), such period of time might be long enough for the
   attacker to correlate all the network activity he is meaning to
   correlate.

   In order to better mitigate network activity correlation and/or
   possibly reduce host exposure, an implementation might want to either
   reduce the preferred lifetime of a temporary address, or even better,
   generate one new temporary address for each new transport protocol
   instance.  However, the associated lifetime/stability of an address
   may have a negative impact on the network.  For example, if a node
   were to employ "throw away" IPv6 addresses, or employ temporary
   addresses [RFC4941] with a short preferred lifetime, local nodes
   might need to maintain too many entries in their Neighbor Cache, and
   a number of devices (possibly enforcing security policies) might also
   need to keep such additional state.

   Additionally, enforcing a maximum lifetime on IPv6 addresses may
   cause long-lived TCP connections to fail.  For example, an address
   becoming "Invalid" (after transiting through the "Preferred" and
   "Deprecated" states) would cause the TCP connections employing them
   to break.  This, in turn, would cause e.g. long-lived SSH sessions to
   break/fail.

   In some scenarios, attack exposure may be reduced by limiting the
   usage of temporary addresses to outbound connections, and prevent
   such addresses from being used for inbound connections (please see
   Section 6).

6.  Usage Type Considerations

   A node that employs one of its addresses to communicate with an
   external server (i.e., to perform an "outgoing connection") may cause
   such address to become exposed to attack.  For example, once the
   external server receives an incoming connection, the corresponding
   server might launch an attack against the aforementioned address.  A
   real-world instance of this type of scenario has been documented in
   [Hein].

   However, we note that employing an IPv6 address for an outgoing
   communications need not increase the exposure of local services to
   other parties.  For example, nodes could employ temporary addresses
   only for outgoing connections, but not for incoming connections.
   Thus, external nodes that learn about client's addresses could not
   really leverage such addresses for actively contacting the clients.



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   There are multiple ways in which this could possibly be achieved,
   with different implications.  Namely:

      Run a host-based or network-based firewall

      Bind services to specific (explicit) addresses

      Bind services only to stable addresses

   A client could simply run a host-based firewall that only allows
   incoming connections on the stable addresses.  This is clearly more
   of an operational way of achieving the desired functionality, and may
   require good firewall/host integration (e.g., the firewall should be
   able to tell stable vs. temporary addresses), may require the client
   to run additional firewall software for this specific purpose, etc.
   In other scenarios, a network-based firewalls could be configured to
   allow outgoing communications from all internal addresses, but only
   allow incoming communications to stable addresses.  For obvious
   reasons, this is generally only applicable to networks where incoming
   communications are allowed to a limited number of hosts/servers.

   Services could be bound to specific (explicit) addresses, rather than
   to all locally-configured addresses.  However, there are a number of
   short-comings associated with this approach.  Firstly, an application
   would need to be able to learn all of its addresses and associated
   stability properties, something that tends to be non-trivial and non-
   portable, and that also makes applications protocol-dependent,
   unnecessarily.  Secondly, the Sockets API does not really allow a
   socket to be bound to a subset of the node's addresses.  That is,
   sockets can be bound to a single address or to all available
   addresses (wildcard), but not to a subset of all the configured
   addresses.

   Binding services only to stable addresses provides a clean separation
   between addresses employed for client-like outgoing connections and
   server-like incoming connections.  However, we currently lack an
   appropriate API for nodes to be able to specify that a socket should
   only be bound to stable addresses.  Development of such an API should
   be considered for future work.

7.  Advice on IPv6 Address Configuration

   [TBD]








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8.  Advice on IPv6 Address Usage

   [TBD]

9.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.

10.  Security Considerations

   This document discusses address usage considerations, and also
   describes possible future standards-track work to allow for greater
   flexibility in IPv6 address usage.

11.  Acknowledgements

   The authors would like to thank [TBD] for providing valuable comments
   on earlier versions of this document.

   Fernando Gont would like to thank Nelida Garcia and Jorge Oscar Gont
   for their love and support, and Ivan Arce and Diego Armando Maradona
   for their inspiration.

12.  References

12.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <http://www.rfc-editor.org/info/rfc4941>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <http://www.rfc-editor.org/info/rfc7217>.



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   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              February 2017, <https://tools.ietf.org/rfc/rfc8064.txt>.

12.2.  Informative References

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
              <http://www.rfc-editor.org/info/rfc7707>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <http://www.rfc-editor.org/info/rfc7721>.

   [I-D.ietf-v6ops-ula-usage-considerations]
              Liu, B. and S. Jiang, "Considerations For Using Unique
              Local Addresses", draft-ietf-v6ops-ula-usage-
              considerations-01 (work in progress), August 2016.

   [I-D.gont-6man-non-stable-iids]
              Gont, F. and S. LIU, "Recommendation on Non-Stable IPv6
              Interface Identifiers", draft-gont-6man-non-stable-iids-00
              (work in progress), May 2016.

   [I-D.gont-opsawg-firewalls-analysis]
              Gont, F. and F. Baker, "On Firewalls in Network Security",
              draft-gont-opsawg-firewalls-analysis-02 (work in
              progress), February 2016.

   [Barnes2012]
              Barnes, R., Altmann, R., and D. Kerr, "Mapping the Great
              Void Smarter scanning for IPv6",  ISMA 2012 AIMS-4 -
              Workshop on Active Internet Measurements, February 2012,
              <https://www.caida.org/workshops/isma/1202/slides/
              aims1202_rbarnes.pdf>.

   [Hein]     Hein, B., "The Rising Sophistication of Network Scanning",
               January 2016, <http://netpatterns.blogspot.be/2016/01/
              the-rising-sophistication-of-network.html>.

Authors' Addresses









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   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com


   Guillermo Gont
   SI6 Networks
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   Email: ggont@si6networks.com
   URI:   https://www.si6networks.com


   Madeleine Garcia Corbo
   Servicios de Informacion del Transporte
   Neptuno 358
   Havana City  10400
   Cuba

   Email: madelen.garcia16@gmail.com






















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